The conveying of fluid medium or power needs to joint a variety of pressure vessels including valves tight by using some pipes or tubes and some pipe or tube fittings to form an enclosed leak-free system. All the fixed tight joints within a pressure vessel and between pressure vessels and pipes or tubes need to be fastened tight, and all the movable tight joints between valving members or discs and valve seats need to be mounted tight; otherwise the controlled transfer of medium or power can not be realized. A fixed or movable tight joint can be made on two parallel flats or two tapered surfaces, on a conical surface to a cone or a sphere, or on a spherical surface to a sphere or a cone; for example, the fixed tight joint of pipe flanges and the movable tight joint of slab gate valves in ISO 14313 are made on two parallel flats, the fixed tight joint between tubes and tube fittings in ISO 8434-2 and the movable tight joint of wedge gate valves in ISO 15761 are made on two tapered surfaces, and the fixed tight joint between tubes and tube fittings in DIN 7601 and the movable tight joint of ball valves in ISO 17292 are made on a conical surface to a sphere. All the fixed tight joints are fastened to be leak free in service, and all the movable tight joints are mounted to be leak free only in stationary closing service and to be movable or not to be leak free for shut-off operations and in stationary open service; i.e. both the fixed tight joint and the movable tight joint are not movable when working as a tight joint, or both are of a static tight joint, and hence the former is especially called a (static) sealing joint or and the latter, a (static) closing joint.
Any machined metallic surface is microscopically of such irregularities that any tight joint can not be made directly on the originally machined metallic surface, and hence any tight joint in the prior art is made either by seating a soft material into or by lapping to eliminate the irregularities on the two jointing machined surfaces.
The tight joint made by a soft nonmetallic material, in addition to bearing some new leaking microchannels in the material bulk, bears many troubles such as relaxation or creep, chemical stability or compatibility, contradiction of sealing ability with jointing strength (because the softer the seating material, the stronger the sealing ability but the weaker the jointing strength and vise verse), thermal stability, etc. caused by seating materials, and hence there exist such a great variety of gaskets of ASME B16.5 Annex C and EN 1514-1˜-8, such a complicated gasket-designing/calculating method of EN 1591, and such complicated gaskets of patents U.S. Pat. No. 6,092,811, U.S. Pat. No. 6,869,081 and U.S. Pat. No. 7,255,353 that it is difficult for an engineer to correctly select a trivial gasket.
Some ring gaskets in ISO 10423 are used for making a sealing joint of two metal to metal machined flange faces, but only disposable. Some metal to metal valve seats in Patents U.S. Pat. No. 4,940,208, U.S. Pat. No. 4,502,663, U.S. Pat. No. 4,262,688, U.S. Pat. No. 4,235,418 and U.S. Pat. No. 4,147,327 were to be used for making a closing joint of two sphere to sphere or cone surfaces, but so far no actual products have been found. A V-toothed gasket in EN 1514-6 and an arc-ridged gasket in Patent application WO 94/29620 are used for making a sealing joint of two metal to metal machined flange faces, but the V-teeth and the arc ridges are macroscopic and shall be covered with soft material to avoid metal to metal contacts.
Obviously, what a simple job it is to design, manufacture, assemble and maintain a tight joint provided directly by two commonly machined surfaces but not by gaskets or lapped surfaces!